74 Introduction to Animal Morphology. 



passing fluid, which they take in after the manner oF 

 Rhizopods, so that each plastide nourishes itself, as 

 do the separate plastides of higher organisms. In 

 rare cases (Guancha blanca), the osculum serves as a 

 mouth, taking in, as well as ejecting, the water 

 (Mikluc1w-Macla\ . The % pores may close, and the 

 osculum diminish in size, by contraction of the sur- 

 rounding plastides. The osculum may be simple, or 

 may have a circlet of lobes or papillae, forming a 

 rudimental tentacular crown, as in Osculina. In some 

 forms, true connective tissue is differentiated beneath 

 the plastide layer in the lining of the central cavity, 

 partly accompanying the canals and partly traversing 

 the sarcode. In the intermarginal cavities of Geodia, 

 fusiform contractile cells, identical with smooth 

 muscle-cells, also exist. 



The surface plastides of many are differentiated 

 into a uni- or multi-lamellar epidermis, often conti- 

 nuous over a whole colony. This layer in Reniera* 

 and Desmacella contains thread cells like those to be 

 described in the next sub-kingdom (Fig. 10). Under 

 this cuticle are often extensive sub-cuticular and inter- 

 marginal spaces, into which open surface pores from 

 without, and the canals of the zooids from within. 



The individuality of a Sponge has been a matter 

 of dispute. Carter and James-Clark regard the 

 " sarcoids" or separate plastides as morphological in- 

 dividuals ; but the arguments in favour of this theory 

 would equally apply to the separate endoplasts of a 

 Ccelenterate, or to [the white corpuscles of the human 

 blood. The view of zooidal individuality stated above 



* Cnidae are also found around the spicules, and especially about the- 

 stomach, in Reniera (Eimer). 



